Posts Tagged 'Larvae'

The sunburst diving beetle, Thermonectus marmoratus, is an adept predator. As adults, these Dytiscid beetles are strong swimmers and prey on a variety of aquatic animals by tearing them to shreds with their powerful mandibles. They also spend some time out of water and can fly from one water supply to another. When it is time to reproduce, female diving beetles enter the water and lay eggs on the stems of aquatic plants and macroalgae. When the eggs hatch, the larvae (known commonly as water tigers) enter the water column and begin their rein of terror.

In the lab, these morphologically distinct diving beetle larvae are typically fed tadpoles or mosquito larvae. In the wild, however, they probably eat anything unlucky enough to get too close. When hunting, these beetle larvae either swim around actively or hang, with their tail touching the surface, just below the water line. When they spot a prey animal, they swim over and strike the target with their powerful mandibles (Watch a video of a predation event below). Unlike the adults, larval diving beetles gradually suck the fluids from their prey, resulting in an unfortunately slow demise.

The predatory nature of sunburst diving beetle larvae is highly dependent on their visual system; and boy is it a bizarre one. While the adults have typical arthropod compound eyes, the larvae see the world through stemmata. Stemmata, which are commonly seen in larval insects, are simple lens eyes that rely on superficially similar optical principles to vertebrate eyes. On each side of the head, the larvae have six stemmata as well as a lens-less eye patch (see below). Within each of these eyes there are two distinct retinas, one on top of another. In total, this means that these T. marmoratus larvae have fourteen eyes and twenty-eight distinct retinas!

Front and side views of the head of a T. marmoratus larva. E, eye; EP, eye patch; M, mandible. Adapted from Mandapaka et al., 2006 and Maksimovic et al., 2009.

This larval visual system has a befuddling number of bizarre optical properties. The retinas are sensitive to a broad range of wavelengths, including UV, and the photoreceptor architecture is suggestive of polarization detection. In addition, some of the lenses seem to have novel bifocal and chromatic aberration-correcting properties. Despite the research into all of these strange visual adaptations, the ecological significance of most of the eyes on this animal is completely unknown.

The best understood eyes in the diving beetle larva are E1 and E2. They are forward-looking and primarily used for predation. However, when you look at the main retina in these eyes, you surprisingly find that it is only composed of a thin horizontal band, two photoreceptors tall. Imagine trying to view the world in a thin strip, two pixels high! So, how is the diving beetle larva using these eyes to zero in on prey? Well, it turns out that these sort of strip eyes are not completely novel in nature. Jumping spiders, some copepods, and a pelagic snail all have strip retinas. In order to see the world, they scan their narrow retinas rapidly back and forth, as in the image below. Diving beetles, on the other hand, have absolutely no musculature to move their eyes or retinas. So how do they see?

Look again at the predation video from above. Notice that once the diving beetle larva spots the mosquito larva, it begins bobbing its entire head up and down. The diving beetle larva is scanning the mosquito with the strip retina in its main eyes. As it gets closer, the scanning movement actually becomes more pronounced, since the target takes up more of the field of view. This technique allows the diving beetle larva to accurately hone in on its prey without sacrificing limited head-space for a full retina or eye muscles.

The closer you examine arthropods, the weirder they seem to get. Who would have though that this small aquatic predator would have such a complex and fascinating visual system? In order to discover the most exciting aspects of living things, you need to look. That’s where science starts; with someone peering through the confounding subterfuge of nature, hoping to widen our glimpse of the gear-works within.

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The research discussed in this post is being carried out at Buschbeck lab at the U. of Cincinnati.

Humans have been aware of the antibiotic properties of some molds and plants for thousands of years. In classical times, fungal molds were used to treat infections. However, the true antibiotic renaissance began in 1928, when Alexander Fleming first isolated penicillin from the fungus, Penicillium notatum. Since then, penicillin and other powerful antibiotics have saved countless lives and greatly assuaged human suffering.

Antibiotics are biologically-produced chemicals that destroy or inhibit crucial components of microbial pathogens, including bacteria, fungi, and protozoans. Penicillin, for instance, works by inactivating the transpeptidase enzyme in Gram positive bacteria, preventing cell wall synthesis, and eventually killing the bacteria. Another antibiotic, Streptomycin, targets the ribosomes of all bacteria, blocking the binding of initiation factors, and preventing protein synthesis. Each class of antibiotic has a fairly unique mode of action and specific target microbes, allowing their use to be tailored on a cases by case basis.

Considering the benefits of antibiotics, it is unsurprising to learn that other organisms have evolved the means of culturing and applying these potent biochemicals. The classic examples are fungus-growing ants (article). These ants, represented by 200 species within the Attini tribe, grow subterranean fungus gardens which they cultivate for nourishment. In addition, they also culture a third symbiote, a filamentous Streptomyces bacterium that produces antibiotics to protect their fungal gardens from parasitic microbes. The ants grow these microbes on their carapaces and pass them on to their offspring.

Beewolves are digger-wasps that consume nectar collected from flowers or from honeybees (Apis mellifera); which they squeeze the nectar out of after paralyzing. Female beewolves dig burrows in the ground and lay their eggs on paralyzed honeybees. When they larvae hatch they consume the bee before climbing to the ceiling of the brood chamber and forming a cocoon.

During the several-month gestation in their cocoons, the beewolf larvae are quite vulnerable to infection by microbes. In order to protect her young, the female beewolf cultures a strain of antibiotic-producing Streptomyces philanthi bacteria within specialized glands on her antenna. Prior to her larvae spinning their cocoons, she secretes her Streptomyces cultures onto the ceiling of the burrow. The bacteria are incorporated into the cocoons as the larvae spin them around themselves. The Streptomyces bacteria then excrete antibiotics into the cocoons, protecting the beewolf larvae from harmful microbes.

Though it was previously shown that beewolves culture Streptomyces to protect their larvae, the nature of the antibiotic protection, provided by the symbiotic bacteria, was unknown. To that end, the current researchers used electrospray ionisation-mass spectrometry and nuclear magnetic resonance spectrometry to identify antibiotics from the beewolf cocoons. Through these ridiculously complicated spectroscopic detection techniques (they may as well be magic as far as I understand them) the researchers identified nine different antibiotic compounds in the cocoons; streptochlorin and eight piericidin derivatives. The researchers demonstrated that these antibiotics where each useful in inhibiting the growth of ten potentially harmful bacteria and fungi microbes. However, the antibiotics were found to be the most efficacious when combined into a complimentary cocktail; as they are found in situ.

The researchers then used imaging mass spectrometry (IMS) to localize the spatial distribution of the three most prevalent antibiotics on the cocoons. IMS works by scanning the surface of an object with an ion beam. This ionizes the chemicals on the object, allowing them to be detected, quantified, and localized with a mass spectrometer. The researchers found that the cocoons had even distributions of the antibiotics over their surface. In addition, they found that the majority of the antibiotics were localized on the outer layer of the cocoon. This led the researchers to hypothesize that the larvae incorporate most of the Streptomyces bacteria early in the spinning process; leaving little left over for the final, internal layers of the cocoon. This has the benefit of keeping the antibiotics on the outside of the cocoon to protect against harmful microbes, while not interfering with the growth of the larvae within.

Beewolves offer a unique case of animals culturing antibiotics for the health of developing individuals. Their antibiotic cocktail approach to microbial control is strongly akin to the synergistic ‘combination therapies’ that are increasingly popular for the treatment of human infections. These techniques have two main advantages: For one, they broaden the effectiveness of the antibiotics to include a wide variety of pathogens. In beewolves, this is advantageous because the developing larvae are threatened by diverse, opportunistic soil and entomological microbes. In addition, antibiotic cocktails are less likely to induce pathogen antibiotic resistance. Against a cocktail, a pathogen would require several simultaneous mutations in order to gain resistance.

The future of human antibiotic treatments are faced with many of the same challenges that the beewolf has risen to meet. In order to solve these problems it is crucial that we also look to nature, as Alexander Flemming did in 1928. Through the trial and error of evolution, beewolves and other organisms have been waging their own antibiotic wars against pathogens for hundreds of millions of years. We would be foolish to ignore their clever solutions to the challenges of surviving on Earth.

The BBC has an audio slideshow of some plankton micro-photography. Plankton is not a taxonomic classification, but rather a bulk term for any pelagic aquatic organism that is typically not capable of out-swimming its currents. (I was recently in a prolonged argument with the owner of salt water aquarium store about this. I could not convince the person that a plankton was not a single type of organism.) Damn near every phylum I can think of has some planktonic species or life stages. You can be plankton if you can’t out-swim the current you are in.

In the BBC video you can see several planktonic crustaceans including adult copepods and euphausiids, as well as the larval stages of crabs. A neat feature to notice in the crab larvae are the long spines protruding from their backs. This is an anti-predation adaptation seen in many crustaceans, making the larvae more difficult to swallow. There is a great diversity of spine length and configuration in different species of larval crustaceans.

Last week I talked a bit about parisitoid fly larvae. Now, the genomes have has been published for three species of parisitoid wasps in Science. These guys are every bit as brutal as the flies, and then some. They forcibly inject their eggs into their insect hosts, often caterpillars. Some species inject multiple eggs while other inject a single egg that later divides into many cloned larvae that form a colonial social hierarchy within their host. Once they mature, the larvae burst out the side of their host. However, even this insult isn’t the end of the torture for the unfortunate caterpillar. The larvae employ mind controlling chemicals to force the caterpillar to use its silk to build them a cocoon, and then watch over its pupating murderers until it keels over dead.

Parisitoid wasp larvae from National Geographic’s, “In the Womb.” Watch in high quality for maximum revulsion.

Nature, you are one psychotic bitch.

Read this review for a wider overview of parasitoid wasps and their contributions to pest control and biological sciences, including discoveries made in the recent genomics work.

One interesting point jumped out at me. The parisitoid wasps are insanely diverse, with species estimates exceeding 600,000. Entomologist Michael Strand even posits that,

There’s a really compelling argument that these parasitoid wasps may be more diverse than beetles[.] Virtually every arthropod on Earth is attacked by one or more of these parasitoid wasps.

This extreme diversity seems to be tied to rapid evolvability (a concept that I have noted before in regards to arthropods) and rigid host specificity. While some wasps are generalists, laying eggs in a variety of arthropods, the vast majority parisitize a specific host species. This specificity is the result of evolutionarily optimized venoms and mind-controlling toxins, tailor made for their host species, and leading to rapid divergence of parisitoid wasp populations as they discover new host animals. This is not unlike the algal specificity of the sea slug I discussed last week. Its biology is ideally suited for maintaining chloroplasts taken up from a single species of algae.

Researchers hope to develop parisitoid wasps as a research model and exploit their prey specificity for pest control that will benefit agriculture. With bio-engineered parisitoid wasp terminators coming soon to a pasture near you, aphids and caterpillars should just cut their losses and run for their lives.

References:
The Nasonia Genome Working Group, 2010. Functional and Evolutionary Insights from the Genomes of Three Parasitoid Nasonia Species. Science, 327(5963), 343-348.